Silyl esters of terephthalic acid as corrosion inhibitors



United States Patent 3,538,000 SILYL ESTERS F TEREPHTHALIC ACID AS CORROSION INHIBITORS Thomas V. Liston, Kenlfield, Calif., assignor to Chevron Research Company, San Francisco, Calif., a corporation of Delaware No Drawing. Filed Nov. 22, 1967, Ser. No. 684,922 Int. Cl. Cm 1/50 US. Cl. 252-495 9 Claims ABSTRACT OF THE DISCLOSURE Silyl esters of terephthalic acid find use as corrosion inhibitors in combination with slightly basic or neutral ashless lubricating oil detergents.

BACKGROUND OF THE INVENTION Field of the invention Corrosion inhibitors are included in modern-day lubricating oils. A large portion of the oxidation products formed in lubricating oils are acidic and corrosively attack such parts of the engine as pistons, piston rings, bearings, etc. Moreover, the water which accumulates can attack iron present in the engine causing corrosion or rusting.

A particularly effective class of corrosion inhibitors, which are popularly used with ashless amine nitrogen containing detergents, are the aromatic dibasic acids. See for example US. Pat. No. 2,809,160 and British Pat. No. 809,198. While these dibasic aromatic acids are very effective as corrosion inhibitors, they are extremely insoluble in hydrocarbon media. Therefore, means for solubilizing the dibasic acids are necessary for their use as corrosion inhibitors.

With the ashless detergents, particularly the alkenyl succinimides of alkylene polyamines, the amines are able to interact with the dibasic acid and act to solubilize the dibasic acid. It is found, however, that when more than 1 amine nitrogen is reacted with an alkenyl succinic acid to form the imide, the resulting dior polysuccinimide or succinamic acid is incapable of solubilizing sufficient aromatic dibasic acid to provide the desired concentration of the dibasic acid corrosion inhibitor,

Description of the prior art The use of aromatic dibasic acids as corrosion inhibitors in lubricating oils is taught in US. Pat. No. 2,809,160 and British Pat. No. 809,198. The interaction of alkenyl succinimide ashless detergents and aromatic dibasic acids is taught in US. Pat. No. 3,287,271.

SUMMARY OF THE INVENTION Pursuant to this invention, silyl esters of aromatic dibasic acids are provided for use as corrosion inhibitors in lubricating oils, particularly in lubricating oils for internal combustion engines compounded with slightl basic or neutral amine nitrogen containing ashless detergents. The dibasic acids are other than ortho-substituted and may have an alkyl substituent of from 1 to 8 carbon atoms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silyl esters which find use in this invention will for the most part have the following formula:

cozsia wherein A is lower alkyl of from 1 to 3 carbon atoms, preferably methyl; R is lower alkyl of from 1 to 8 carbon atoms, more usually 1 to 4 carbon atoms; and n is an interger of from 0 to 1. The carboxyl groups are separated by at least 3 annular carbon atoms. The preferred composition is the di(trimethyl silyl) ester of terephthalic ac1 Other dibasic acids which may be used are the silyl esters of isophthalic acid, tert.-butyl isophthalic acid, tert.- butyl terephthalic acid, methyl terephthalic acid, etc.

As already indicated, the silyl esters find particular use with amine nitrogen containing detergents which are incapable of solubilizing a sufiicient amount of the nonesterified dibasic acid for use in a lubricating oil. Normally, in a lubricating oil composition, the detergent will be present in from 2 to 10 weight percent, more usually 2 to 8 weight percent. About 0.075 to 0.30 weight percent based on the total composition of the dibasic acid (includes ester derivative) will be used, usually about 0.10 to 0.2 weight percent of the dibasic acid will be used. Therefore, when less than about 0.05 weight percent of the total composition of the dibasic acid can be solubilized, the amount of dibasic acid must be augmented in order to maintain a satisfactory level of corrosion inhibition.

In lubricating oil concentrates, the detergent will be present in from about 10 to 40 weight percent and concomitantly the total dibasic acid will be present in from 0.5 to about 1.2 weight percent of the total composition.

The ashless detergents are those combining an alkylene polyamine having from 2 to 6 nitrogen atoms with at least 1.8 moles of an alkenyl succinic acid per mole of alkylene polyamine. The product of the reaction may be a succinamic acid, a disuccinamide, or succinimide, or combinations thereof. These compositions are prepared by heating from 1.8 to 3 moles of an alkenyl succinic anhydride, usually a polyisobutenyl alkenyl succinic anhydride, having a molecular weight in the range of about 500 to 2,000 with an alkylene polyamine, usually an ethylene polyamine or propylene polyamine having from 2 to 6 amine nitrogen atoms at a temperature in the range of C. to 200 C. for a time sufficient to cause reaction. Any water formed during the reaction. is driven off to drive the reaction to completion. These detergents are described in US. Pats. Nos. 3,219,666 and 3,172,892, the disclosures of which are incorporated herein by reference.

Of particular interest are the polyisobutenyl succinimides of triethylene tetramine and tetraethylene pentamine, wherein the polyisobutenyl groups are of from about 700 to 1.500 molecular weight and the mole ratio of polyisobutenyl succinic anhydride to alkylene polyamine is in the range of 1.8 to 2.2.

While the silyl esters may be compounded with the ashless detergent, without the addition of the parent dibasic acid, it is more economical to dissolve the maximum amount of dibasic acid in the oil which can be solubilized and then augment that amount with the silyl ester. Usually, the total concentration of parent dibasic acid and its silyl ester derivative will be at least 0.075,

more usually at least 0.10 weight percent and generally not more than 0.3 weight percent, more usually not more than 0.15 weight percent. Preferably, a total of about 0.1 weight percent of the oil composition is preferred. The concentrate will be accordingly multiplied in relation to the detergent.

Usually, the parent dibasic acid will be present in from 0.01 to 0.05 weight percent, while the silyl ester will be present in at least 0.05, preferably 0.075 to 0.29 weight percent.

The lubricating oils which find use are hydrocarbonaceous oils of lubricating viscosity, particularly petroleum oils, which may be naphthenic base, paraffin base, asphaltic base and mixed base lubricating oils. Also, synthetic oils such as polymers of hydrocarbon olefins, alkyl aromatics, etc. may be employed.

The silyl esters of this invention may be prepared by any convenient means. Conveniently, a (trialkyl silyl) amine or di(trialkyl silyl)amine may be combined with the desired dicarboxylic acid and heated at elevated temperatures either neat or in an inert solvent. Reference to the method of preparation of silyl esters may be found in C. Eaborn, Organic Silicon Compounds, Academic Press, Inc., New York, N.Y., 1960, Chapters 9 and 11.

The following example is offered by way of illustration and not by way of limitation.

EXAMPLE I Into a reaction flask was introduced 7 g. of terephthalic acid and 25 cc. of di(trimethyl silyl)amine and the mixture heated under nitrogen for 3 hours. After allowing the mixture to cool, long needle-like crystals formed. The mixture was filtered, the crystals dissolved in hexane, the hexane solution filtered and the hexane distilled off. The residue weighed 11.2 g. An infrared spectrum of the product indicated that the desired di(trimethyl si1yl)ester of terephthalic acid had been obtained.

Using a highly hindered polyisobutenyl succinimide of tetraethylene pentamine (approximately 2 succinimides per tetraethylene pentamine; the polyisobutenyl group being of about 1,000 molecular weight) the silyl ester of terephthalic acid was tested as a corrosion inhibitor in an L-38 strip test. Copper and lead strips are immersed in oil formulations using a 480 neutral oil at a temperature of 340 F. for hours and the weight loss determined at the end of this time. The following table indicates the results obtained.

1 The strip is washed with dilute potassium cyanide solution (cone. wt. percent in 1120) to remove copper salts.

To further test the compositions of this invention, engine tests were performed. An L38 test using a l-cylinder CLR engine was carried out for 40 hours at an engine speed of 3,150 r.p.m. The results are reported as the bearing weight loss in milligrams. The first compounded oil (A) prepared had 9.54 weight percent of the detergent described above and 0.022 weight percent of terephthalic acid, the maximum amount which could be solubilized. The second compounded oil (B) added to the first oil, the composition of Example I, in an amount to provide 0.16 weight percent of the lubricating oil composition. For (A) there was a bearing weight loss of 303 mg, while for (B), the loss was only 119 mg.

It is evident from the above results, that the silyl esters are eflective corrosion inhibitors in lubricating oil in combination with an ashless amine-containing detergent. Moreover, because of the silyl ester solubility in the lubrieating oil, a suflicient amount of the corrosion inhibitor can be maintained in the oil to provide adequate protection in an internal combustion engine.

I claim:

1. A lubricating oil composition having an oil of lubricating viscosity from 0.05 to 0.29 weight percent of a silyl ester of the formula:

CO2SiA wherein A is lower alkyl of from 1 to 3 carbon atoms, R is lower alkyl from 1 to 8 carbon atoms, n is an integer of from 0 to 1 and the carboxyl groups are separated by at least 3 annular carbon atoms.

2. A lubricating oil composition according to claim 1 having from 2 to 10 weight percent of an ashless detergent prepared by combining an alkylene polyamine having from 2 to 6 nitrogen atoms with from 1.8 to 3 moles of an alkenyl succinic anhydride having a molecular weight in the range of 500 to 2,000.

3. A lubricating oil composition according to claim 2 containing terephthalic acid in the maximum amount which can be solubilized by said ashless detergent.

4. A lubricating oil composition according to claim 1 wherein A of the silyl ester is methyl.

5. A lubricating oil composition according to claim 1 wherein the carboxyl groups of the silyl ester are para,

6. A lubricating oil composition according to claim 1 wherein R of the silyl ester is tert.-butyl and n is 1.

7. A lubricating oil composition according to claim 6 having from 2 to 10 weight percent of said composition of a polyisobutenyl succinimide obtained by reacting triethylene tetramine or tetraethylene pentamine with a polyisobutenyl succinic anhydride having a molecular weight in the range of 700 to 1,500 and wherein said mole ratio of said anhydride to the amine is in the range of 1.8 to 2.2.

8. A lubricating oil composition according to claim 1 wherein the silyl ester is present in an amount from 0.075 to 0.29 weight percent and wherein the carboxyl groups of the silyl ester are para.

9. A lubricating oil composition having an oil of lubricating viscosity and from 0.5 to 1.2 weight percent of a silyl ester of the formula:

C OZSIA3 COZSIAa wherein A is lower alkyl of from 1 to 3 carbon atoms, R is lower alkyl from 1' to 8 carbon atoms, n is an integer of from 0 to 1 and the carboxyl groups are separated by at least 3 annular carbon atoms.

References Cited UNITED STATES PATENTS 2,324,770 7/1943 D'onlan 252389 X 2,809,160 10/1957 Stewart et a1 25242.7 X 3,287,271 11/1966 Stuart et a1.

FOREIGN PATENTS 809,198 2/1959 Great Britain.

DANIEL 'E. WYMAN, Primary Examiner W. J. SHINE, Assistant Examiner US. Cl. X.R. 252-51.5, 389 

